Type XXI snorkel was cleared for 10 kts submerged cruise (10.42 kts in trials).
Type XXIII snorkel was cleared to 12 kts submerged cruise.
Note that at these levels of speed, no charging of the batteries is possible and there may even be electric motors switched on to augment the Diesels. Parctical recharging speeds varied between 6 and 7.5 kts for XXI and 5 to 7 kts for XXIII. At higher speeds than 6 kts, the improved sonar was disfunctional, too.
So my assumption is that nobody would dare to snort an extensive time at maximum submerged cruise speed on Diesels. The pume of smoke isn´t visually invisible, the amount of sound generated creates a noisy signal and can be detected from quite some distance. Another problem is the wave created by the snorkel. While the Wesh-coated snorkel head itselfe may be close to invisible (within the ground clutter return) to radar detection, the wave created at high snorting speed is not invisible and microwave radar sets could detect and discriminate wave signals, particularely at very calm Seas (in the 70´s it was seastate +1 kts and You are safe, indicating very slow snorting speeds).
Thus, I don´t expect a lot of change.